As is well known in the art, a wind turbine is an apparatus that converts the energy of the wind into useful electrical energy. The wind turbine generates electricity using the rotary force of blades, the motion being produced when the wind rotates the blades. Since the wind turbine uses the wind, which is an unlimited clean source of energy, as a power source to perform non-polluting power generation, the effect achieved by substituting it for fossil fuel is great. In addition, by installing the wind turbine in under-developed areas, coastal areas, and mountainous areas, it is possible to rationalize the use of the land of a country and supply competitive power. Further, when a massive wind power plant complex is constructed in a specific region, such as an island, the wind turbines can also serve as tourist attractions. Therefore, the wind turbine is attracting more attention.
Since the wind turbine has been constructed in the form of a ‘rotor blade type’ (the so-called propeller type) wind turbine by the Danish physicist Poul la Cour in 1891, it is currently in the spotlight and is increasing in size. In addition, in wind power generation, the output of the wind turbine varies depending on the conditions of its construction. For example, the strength of the wind and the size of a wind turbine are very important factors, since more wind energy can be produced when the wind is faster and the wind turbine is larger. In addition, locating a wind turbine higher is better and generates more power than a lower wind turbine does because the wind becomes stronger as the height increases. Wind blowing at an average velocity of 4 m/s or more is required in order to use wind to generate electric power. Herein, the velocity of the wind refers to its velocity at the height at which the blades of the wind turbine are present not its velocity on the ground that people stand on.
Such wind turbines are classified according to the direction of the rotary shaft of the blades, into a vertical-axis wind turbine, in which the rotary shaft is provided perpendicular to the ground surface, and a horizontal-axis wind turbine, in which the rotary shaft is provided parallel to the ground surface. The horizontal-axis wind turbine is easy to construct because of a simple structure. However, the horizontal-axis wind turbine is greatly influenced by the wind. Although the vertical-axis wind turbine can be constructed in a desert or plain regardless of the direction of the wind, its efficiency is disadvantageously lower than that of the horizontal-axis wind turbine.
FIGS. 1A and 1B show an example of a rotor blade type wind turbine having a typical horizontal-axis structure. As shown in FIGS. 1A and 1B, the rotor blade type wind turbine includes a rotor 10, which converts wind power into mechanical rotation energy, a nacelle assembly 20, which includes components for converting the rotation energy into electrical energy, and a tower 30, which supports the nacelle assembly 20. The wind turbine is completed by burying a foundation insert 40, which is supposed to be under the tower 30, in the location in which foundation work is firmly finished, and sequentially assembling the tower 30, the nacelle assembly 20, and the rotor 10 over the foundation insert 40. The rotor 10 includes a hub-nose cone assembly 14, which includes a plurality of blades 12, for example, three blades, which are arranged radially at equal intervals. The hub-nose cone assembly 14 is connected to the main shaft 22, which is supported on a base frame 24 inside the nacelle assembly 20. A speed-up gearbox 26, a disc brake 28, and a dynamo 50 are assembled sequentially to the main shaft 22. The blades 12 are disposed in an orthogonal direction to the main shaft 22, and therefore the hub-cone assembly 14 rotates when the wind blows the blades. This rotating force is transferred to the main shaft 22, and the number of rotations of which is increased by the speed-up gearbox 26, thereby driving the dynamo 50 that generates power.
In wind power generation, it is most preferable that a so-called free yaw state be realized, since the availability of wind energy is high when the plane on which the blades rotate (i.e., the rotating plane of the blades) intersects the direction of the wind at right angles. However, since the direction of the wind changes constantly, there occurs a yaw error in which the rotating plane of the blades no longer intersects the direction of the wind and deviates at right angles. As the yaw error becomes greater, the availability of the wind drops.
In order to prevent this problem, the wind turbine also includes an active yawing system 60, which is provided in the nacelle assembly 20, as specifically shown in FIG. 2. The active yawing system 60 includes a ring gear 62, which is mounted on a top flange formed on the upper end of the tower, and a wind direction control motor 64, which interlocks with the ring gear 62. When the direction of the wind changes, the wind direction control motor 64, interlocking with the ring gear 62, is operated to rotate the nacelle assembly 20, thereby realizing active yaw control. Accordingly, the blades 12 are operated to constantly face the wind. In the figures, reference numeral 66 indicates an anemoscope.
FIG. 3 shows a rotor blade type wind turbine having a typical horizontal structure in which a dynamo is installed on a ground or below a tower. As shown in FIG. 3, the nacelle assembly 20 is supported on the upper portion of the tower 30 by a bearing assembly 82. In this state, the rotating force of the main shaft 22 is transferred through a drive bevel gear 72a to the vertical tower shaft 76 having a following bevel gear 74a, which is engaged with the drive bevel gear 72a, and is then transferred through a following bevel gear 72b to a speed-up gearbox 26 via a rotating shaft 78. The speed-up gearbox 26 is used to drive the dynamo 50. With this configuration, the dynamo 50 may be provided on the ground or at a predetermined height not far above the ground. However, in this case, when the rotating force of the blades 12 of the rotor 10 is transferred through the drive bevel gear 74b, which is coupled to the lower end of the vertical tower shaft 76, and through the following bevel gear 72b of the rotating shaft 78, the tower shaft 76 is subject to repulsive torque (shown as dotted lines) that is applied to the drive bevel gear 74b from the following bevel gear 72b. The repulsive torque causes the nacelle assembly 20 to rotate. Therefore, in order to preclude the repulsive torque, a strong rotation prevention function has to be disadvantageously added to the inside of the active yawing system. Accordingly, the wind turbine is generally provided inside the nacelle assembly instead of being provided on the ground or at a position close to the ground even if the cost increases.
In general, in the case of a megawatt level wind turbine, the tower is designed to be approximately from 50 to 80 m in height in consideration of the direction of the wind and other factors. In addition, the tower is required to support a total tower head mass of substantially 100 tons, i.e., the load of the nacelle assembly, including the dynamo and the rotor on the upper portion thereof. Therefore, the tower has to be designed to have a structural strength meeting such conditions, and the top flange of the tower is large, the outer diameter thereof being nearly 3 m, which entails an increase of construction and maintenance costs.
In addition, since the blades are linear, a strong reaction increases energy loss, and the rotating blades cause a large amount of aerodynamic loss. In the case of a down wind, a low pressure area is formed in the backwash of the tower, thereby increasing noise and causing fatigue to the blades.